Gastroesophageal reflux disease (GERD) is a medical condition that affects approximately 15% of adult population in the United States and is one of the most prevalent clinical conditions afflicting the gastrointestinal tract. GERD refers to symptoms or tissue damage caused by the reflux of stomach contents into the esophagus and pharynx. The most common symptom of GERD is heartburn and acid regurgitation. GERD has been associated with esophageal cancer and chronic lung damage. Two common esophageal cancers are squamous cell carcinoma and adenocarcinoma. In the United States, esophageal carcinoma accounts for 10,000 to 11,000 deaths per year. Adenocarcinoma of esophagus has the fastest growing incidence rate of all cancers. These increased rates are strongly related to GERD which is the primary risk factor recognized [1]. Therefore, monitoring the GERD symptoms comfortably and reliably becomes more important for early diagnosis of esophageal cancer.
While pH testing has been used to detect acid reflux, esophageal impedance monitoring is a new technique that can detect episodes of gastroesophageal reflux that are both acidic and non-acidic in nature. This technique overcomes the limit of ambulatory pH-metry which does not always reliably detect the reflux of material whose pH value is more than 4.0 [2]. Multichannel intraluminal impedance (MII) probe is a currently available instrument that has been used to correlate symptoms with episodes of gastroesophageal reflux. Whereas electric conductivity is directly related to the ionic concentration of the intraluminal content, materials with high ionic concentrations (e.g. gastric juice or food residues) have relatively low impedance compared with that of the esophageal lining or air [3]. Although the MII probe system brings more accurate monitoring results compared to the conventional pH meter alone, the configuration is bulky and uncomfortable for patients. The tethered sensor probe requires a transnasal insertion procedure and the wire, connecting from the electrodes that stay inside the esophagus to the external electronic unit worn by the patient, stays transnasally for 24 to 48 hours while the patient supposedly resumes normal daily activities. The wired feature limits the clinical utility and accuracy of this technique for protracted monitoring of gastroesophageal reflux. A miniature wireless device that does not require tethered external connections is thus preferred for esophageal reflux monitoring.
To date, a wireless pH monitoring capsule (BRAVO, Medtronic) has been used in some clinical practices [4]. However, it cannot detect non-acid reflux and has a limited battery life. Recent studies and reviews have suggested combined pH and impedance monitoring increased the accuracy of GERD diagnosis [5, 6]. Lately, a combined impedance and pH sensor capsule that could detect both acid and no-acid reflux was developed using a microcontroller and a wireless transmitter [7]. However, the device has limited sampling rates to conserve battery energy. The limited sampling rate may miss reflux episodes between sampling. The limited battery lifetime prohibits the possibility of prolonged measurements that in some clinical cases are needed for increased diagnosis accuracy [8]. Although batteryless wireless approaches for communication of implantable devices have been proposed [9, 10], they are not currently utilized for reflux diagnosis using an impedance to frequency converter.
Accordingly, there remains a need for an improved system that accurately monitors a patient's gastroesophageal acid reflux that is more compact, untethered, improves patient comfort, and does not depend upon an implanted power source for its function.